This invention relates generally to circuits having active loads and more particularly to biasing circuits used therein.
As is known in the art, monopulse antenna systems are widely used in radar systems to track targets. Such antenna systems generally include four antenna elements disposed in a four quadrant array. The signals received by the four antenna elements are fed to an arithmetic unit. The arithmetic unit combines the received signals to produce a sum antenna pattern, an azimuthal difference antenna pattern, and an elevation difference antenna pattern. The sum antenna pattern is produced by adding the signals received by the four antenna elements. The azimuthal difference antenna pattern is produced by adding the sum of the signals received by the antenna elements in the second and third quadrants and subtracting such sum from the sum of the signals received by the antenna elements in the first and second quadrants. The elevation difference antenna pattern is produced by adding the sum of the signals received by the antenna elements in the first and second quadrants and subtracting such sum from the sum of the signals received by the antenna elements in the third and fourth quadrants.
As is also known in the art, the arithmetic unit includes an arrangement of passive microwave devices, such as magic tee, branch lines, hybrid junctions, or rat races. Such arrangements are described in Introduction to Radar Handbook by Merrill I. Skolnik pages 177 to 178, published by Mc-Graw Hill Book Company, Inc., New York, New York, 1962. In some applications, the monopulse arithmetic unit operates at an intermediate frequency below X-Band. One such application is described in U.S. Pat. No. 4,980,925, "Monopulse First Detector Array", invented by Martin R. Blustine et el, issued Dec. 25, 1990, and assigned to the same assignee as the present invention. While the passive microwave devices described above provide the requisite sum and difference antenna patterns they are relatively large, particularly at lower microwave frequencies (i.e. below X-Band). Thus such arrangements do not lend themselves to implementation as microwave monolithic integrated circuits (MMIC).
As is also known in the art, microwave monolithic integrated circuits have been used in a wide variety of applications. Such circuits integrate on a single substrate, typically gallium arsenide, passive and active microwave components. One such active device is a microwave amplifier. As is further known in the art, analog circuits have a wide variety of applications such as in analog to digital converters, digital to analog converters, frequency to voltage converters, etc. One analog circuit is a differential amplifier. As is known, one such differential amplifier is a single-ended output differential amplifier. With such arrangement a voltage is produced at the single output which is proportional to the difference in voltages fed to a pair of input terminals of the amplifier. While ideally the output voltage is zero if the voltages fed to the input terminals are equal, in any practical circuit a residual voltage is produced at the output because of imbalances in the amplifier. The amplifier produces an output voltage having two components: a component proportional to the sum of the two input signals, called the common mode signal, S.sub.C, and a component proportional to the difference between the pair of input signals, called the differential mode signal, S.sub.D, where S.sub.D is the product of S.sub.C and the common mode rejection ratio (CMRR). The larger the CMRR, the more ideal the differential amplifier, all other performance parameters being equal.
One method used to increase the CMRR of a differential amplifier operating at microwave frequencies is to use a second amplifier as an active balun by, in effect, cascading two differential stages. However, the size of the resulting circuit is relatively large and the DC power consumption increases with each stage.
As is also known in the art, one type of differential amplifier includes a pair of transistors, with each transistor having a pair of electrodes coupled across a power supply. Input signals are fed to the control electrodes of the transistors. Sometimes passive, i.e. resistive or inductive, loads are coupled between the transistors and the power supply for biasing the transistors to their proper operating region. Use of resistive loads, however, increase the required operating voltage as compared with the use of active loads. Inductive loads require large amounts of surface area and limit the low frequency performance of the circuit. While the use of active loads has been suggested, errors in the process of forming the transistors may result in mismatched devices. Adjustment in the DC biasing levels to compensate for such mismatches after fabrication is difficult to achieve and impractical in high volume production.